The switched mode power supply (SMPS) is a well-known type of power converter, which has a diverse range of applications by virtue of its small size and weight, and also its typically high efficiency. For example, switched more power supplies are often used in personal computers and portable electronic devices such as cell phones. An SMPS achieves these advantages by switching a switching device such as a power MCSFET at a high frequency (usually tens to hundreds of kHz), with the frequency or duty cycle of the switching being adjusted using a feedback signal to convert an input voltage to a desired output voltage. An SMPS may take the form of a rectifier (AC/DC converter), a DC/DC converter, a frequency changer (AC/AC) or an inverter (DC/AC).
FIG. 1, is a block diagram illustrating some of the key components of a typical DC/DC converter, which converts an input DC voltage Vin to a desired DC output voltage Vout. The primary side of the power supply 100 comprises a switching device in the form of a power transistor 110, whose switching generates an AC voltage from the input voltage Vin. The AC output of the transistor 110 drives a primary winding of the power supply's transformer 120, causing an AC current to be induced in the transformer's secondary winding. The AC voltage thus generated across the transformer's secondary winding is filtered by a filter 130, which typically comprises an inductor coupled to a capacitor in a low-pass filter configuration (not shown).
The power supply 100 further comprises a feedback circuit in the form of a voltage regulator 140, which is arranged to monitor the power supply's output voltage Vout and generate a feedback signal on the basis of Vout and a reference voltage, Vref. The feedback signal can be regarded as an error signal indicative of the difference between the power supply's output voltage (and the target voltage or set-point, Vref. In particular, the voltage regulator 140 is of an integral wind-up type in the present example, such that the feedback signal increases as a (long-term) integrated voltage error, which is accumulated by the voltage regulator, increases. Although information may be encoded in the feedback signal by frequency- or phase-modulating a carrier wave, it is preferable for information to be encoded in the feedback signal simply via the amplitude of a DC voltage. The voltage regulator can be provided in the form of a PID, PI or I-controller, for example.
The feedback signal is communicated from the voltage regulator 140 via an isolation barrier 145 (e.g. an optical coupler) to the power supply's switching controller, which is provided in the form of a pulse width modulator (PWM) 150. As illustrated in FIG. 1, the PWM 150 on the primary side is configured to generate and apply voltage pulses 155 of an appropriate frequency (e.g. 100 kHz) to the gate of the power transistor 110. Moreover, the PWM 150 is arranged to vary the output voltage Vout by adjusting the duty cycle D of the pulses (defined by D=TON/Ts, where TON is the duration of a pulse and Ts is the switch period) based on the received feedback signal. Alternatively, instead of the PWM controller 150, a frequency-modulating controller (not shown) can be used, which modulates the frequency at which pulses of a fixed duration are generated.
The power supply 100 also includes an output current monitor 160 which is arranged to measure the power supply's input current and communicate the results of its current measurement to a Module Management Controller (MMC) 170 of the power supply, which is also provided on the primary side of the power supply. The MMC 170 is preferably further configured to receive other parameters relating to the operation of the power supply, such as information concerning the duty cycle set in the PWM 150, the temperature in the transformer 120 (and/or the temperature at another hot-spot in the power supply), system status information for fault monitoring and diagnostics etc. These parameters may be used by the MMC 170, for example to implement safety features such as protective cut-offs which ensure that critical parameters such as the component temperatures do not exceed pre-determined thresholds. The MMC 170 may forward some of the received information to a higher-level system which may be located off the board(s) on which the converter 100 is formed.
In the present example the MMC 170 serves to configure, monitor and control operational parameters and settings of the converter 100. For example, the MMC 170 may be configured to control a start-up procedure performed by the power supply, during which the converter gradually transitions from a non-operational state (in which the power supply transfers no power from its input to its output) to operating in a first or a second operational mode. These modes will now be described.
During operation of the converter, the current monitor 160 generates a signal indicative of the input current level and processes the monitored signal by filtering it. The filtered signal is then communicated to the MMC 170. Alternatively, the aforementioned signal processing may be performed in the MMC itself. In either case, the MMC 170 then determines whether the received signal indicates that the power supply's input current has exceeded a threshold value.
During normal operation, where the power supply's input current is below the threshold and thus within a safe operating range (such that the power supply can operate for extended periods of time without sustaining damage), the feedback control loop comprising the voltage regulator 140 and the PWM 150 maintains the output voltage Vout at a predetermined level (for example, the CPU core voltage of a CPU that the converter powers) by making small corrections to the duty cycle of the transistor switching. During this first mode of operation, the feedback signal generated by the voltage regulator 140 will generally be very small.
If, on the other hand, the MMC 170 determines that the power supply's input current has exceeded the threshold value, the MMC generates an excess current control signal for controlling the PWM 150 to progressively decrease the duty cycle in order to reduce the output voltage Vout, and thus bring the input current to within the safe range. That is, when the output current exceeds the Threshold, the PWM 150 operates in a second operational mode (also referred to herein as the “continuous current protection” (CCP) mode) to reduce the output voltage Vout, and therefore the input current, to below the threshold, on the basis of the excess current control signal generated by the MMC 170. Thus, since CCP has priority over voltage regulation, in the CCP mode the PWM 150 allows the excess current control signal from the MMC 170 to override the feedback signal from the voltage regulator 140, allowing the PWM to implement a (often linear) current limiting function. In this way, the converter will attempt to limit its output power.
The output current monitor 160 and the MMC 170 together provide the function of a current limiting circuit, which determines the input current and generates an excess current control signal when the input current exceeds a current threshold. During operation in the CCP mode, the excess current control signal will depend on the size of the deviation of the output current from the threshold but will generally be very small. In practice, operation in the CCP mode provides something between constant current and constant power control, depending on whether the converter is a current mode converter (often used in a Flyback configuration) or voltage mode converter (often used in half-bridge or full-bridge configurations).
The over-current protection (OCP) provided by the above-described power supply operating in the CCP mode is effective where the excursions of the input current above the threshold are small and short-lived. The power supply will be able to recover smoothly from such over-current situations using CCP.
However, operation in the CCP mode gives rise to a long current tail, which means that the power supply could, under some load conditions, operate at a high enough load level and for a sufficient period of time to overheat or sustain damage. Robustness against such overheating or damage may be improved by increasing the power supply's input current headroom, although at the cost of reducing its power rating. Moreover, a more extreme over-current situation could occur during operation, such as a crowbar short-circuit, in which CCP would be ineffective.
In view of these shortcomings of CCP, power supply designers have resorted to other, more aggressive protective measures instead of CCP, such as latching the power supply's output when the input current exceeds a threshold, thereby losing the advantages of CCP. In other words, in this scheme the converter is stopped when an over-current situation is detected, and then needs to be reset externally by the user. A related approach employs the so-called “hiccup” functionality, whereby the converter itself attempts to restart after being stopped, rather than having to be restarted externally.
Although these types of OCP can respond very quickly to protect the converter against rapid load changes (e.g. a crow-bar short-circuit), they cannot distinguish between such rapid load changes and more benign ones, which could be handled without shutting down the converter, for example by using CCP. Besides the risk of false triggering, the output-latching and hiccup approaches require a guard band to be provided up to the trigger point, which leads to the further risk of the converter sustaining damage by operating within the guard band for a lengthy period of time.
In these alternative approached to OCP, the input current needs to be monitored by the current monitor 160 and a decision reached on whether a shut-down or a restart of the converter is justified. However, the input current monitored by the current monitor 160 in the converter of FIG. 1 is small and noisy, and would therefore need to be filtered and amplified before it can be analysed. However, providing the current monitor 160 or the MMC 170 with the required additional filtering and amplification means significantly increases the component count and the fabrication cost of the converter, and since these additional components will inevitably dissipate power, also lowers the converter's efficiency.
Accordingly, there has been a need to develop an efficient SMPS of a simple construction having an OCP mechanism that retains the advantages of conventional CCP whilst providing second-level protection against current level increases that cannot be handled safely by CCP.